The stuff you scrape off burnt toast is made primarily of atoms of carbon.
But what makes up a carbon atom -- or any other atom?

The first subatomic particle to be identified was the electron, in 1898.
Ten years later, Ernest Rutherford discovered that atoms have a very dense nucleus, which contains protons.
In 1932, James Chadwick discovered the neutron, another particle located within the nucleus.

And so scientists thought they had found the smallest atomic building blocks.
This changed in 1963 when Murray Gell-Mann proposed his quark theory.
Gell-Mann believed that each proton and each neutron is made up of three even smaller particles -- particles he named quarks.

Physicists have learned a great deal over the past 100 years. For instance, it is now known that in each atom of carbon12,
there are a set number of subatomic particles: six electrons, six protons, and six neutrons.
The atom's nucleus and electrons are held together by the electromagnetic force --
the postitive charges of the protons balances the negative charges of the electrons.
Neutrons have no charge.

Here's a chance for you to construct a carbon atom. You'll start with a hydrogen atom,
which contains one proton and one electron. Just add protons, neutrons, and electrons.
By the way, you must also build each proton and neutron from two types of quarks -- up quarks and down quarks.
Finally, some advice: try to keep the particles' charges balanced. You'll never succeed if you don't.